Gear differentials generally include compound planetary gear sets interconnecting a pair of drive axles to permit the latter to rotate in opposite directions with respect to a differential housing. The drive axles rotate about a common axis; and a pair of respective side gears (sometimes called "sun" gears) are fixed for rotation with the inner ends of the two drive axles, such side gears acting as the sun gear members of the compound planetary gear sets. The side gears are interconnected by planet gears sometimes called "element" or "combination" gears. The planet gears are usually arranged as sets of meshing pairs, being spaced circumferentially and equidistant about the common axis of the sun gears (e.g., four pairs arranged at 90.degree. intervals or three pairs at 120.degree. intervals); and the planet gears may be mounted for rotation about axes that are variously offset and inclined with respect to a common axis of the sun gears and drive shafts. My invention relates primarily to "parallel-axis" differentials in which the planet gears are mounted on axes parallel to the common axis of the sun gears.
The entire planetary gearing arrangement within the differential housing supports opposite relative rotation between the drive axle ends (i.e., differentiation), which is necessary to permit the axle ends to be driven at different speeds. Torque transmitted to the drive axles through the inclined tooth surfaces of the sun/side gears generates thrust forces against gear-mounting bearing surfaces within the differential. (Such bearing surfaces may comprise journals formed in the housing, or may be the bores or the ends of the bores into which the gears are received, or may be special washers positioned between the end faces or shaft ends of the gears and the housing.) The thrust forces, together with other loads conveyed by the gear meshes in the planetary gearing, produce a frictional resistance to relative rotation between the drive axles, this frictional resistance being proportional to the torque applied to the differential housing. The proportional frictional resistance supports different amounts of torque between the two drive axles to prevent their relative rotation until the characteristic "bias" ratio of the planetary gearing arrangement is reached. Once the frictional resistance is overcome and differentiation begins, the torque difference between the axles is proportioned in accordance with the bias ratio. Differentials that divide torque in a substantially constant ratio between relatively rotating drive axles are referred to as "torque-proportioning" differentials.
The ability to support different amounts of torque between the drive axles is of great benefit to improving traction capabilities of vehicles. Ordinarily, when one wheel of a vehicle with a conventional differential loses traction, the amount of torque that can be delivered to the other drive wheel is similarly reduced. However, when one wheel of a torque-proportioning differential loses traction, an increased amount of torque is delivered to the drive wheel having better traction, such increased torque being determined by the characteristic bias ratio of the differential.
In typical parallel-axis torque-proportioning differentials (e.g., U.S. Pat. Nos. 2,269,734 to L. S. Powell and 3,706,239 to A. F. Myers), each planet gear is in mesh with a paired planet gear; each planet gear in the pair meshes, respectively, with one of the sun gears; and one axial end of each individual planet gear is in mesh with its respective side gear, while its other axial end is in mesh with its paired planet gear. This common form of planetary gearing is appropriate for some embodiments of my invention, but preferred embodiments utilize unusually shaped planet gear pairs which are referred to as "straddle" gears. These preferred straddle planet gears mesh with each other at two or three separated engagement areas, and two of these engagement areas on each planet gear straddle the portion of the gear which is in mesh with its respective side gear. (See U.S. Pat. No. 5,122,101 to G. B. Tseng and my U.S. patent application Ser. No. 327,027, filed 2 Oct. 1994.)
In regard to one of the features of the invention, a significant portion of automobiles presently being manufactured throughout the world use so-called "C-clips" for assuring that the axle ends cannot be accidentally withdrawn from the differential (see U.S. Pat. No. 4,365,524 issued to Dissett et al.). In this well-known type of assembly, C-shaped (i.e., partial ring)fasteners are fitted within annular grooves formed near the axle ends after the latter have been inserted through respective journals formed in the differential housing and through a respective one of the sun/side gears.
In order to complete this C-clip assembly, it is necessary to provide space for some relative motion between each axle end and the differential housing so that each axle end can be inserted within the differential case for a sufficient distance to expose the locking ring groove formed in the axle end. Once the C-clip locking ring is installed in place, the axle part is then withdrawn to a desired position for normal driving operation. After this has been done for each respective axle part, it is necessary to insert some means for preventing further axial movement of the axles to maintain them and their respectively captured C-clips in the desired position.
Accommodation for C-clip assembly requires that sufficient space be available within the differential housing to permit the insertion and attachment of the C-clips to the axle ends, and this space requirement has traditionally been met with existing higher-bias parallel-axis designs by the removal of at least one set of the differential's planetary gear pairs. Known designs of parallel-axis differentials cannot afford to lose such a gear set. That is, the loss of such planetary gearing reduces the differential's available torque capacity below the levels specified for its appropriately practical torque-proportioning use.
Of course, known designs could be significantly enlarged to provide the space requirements of C-clip assembly between existing planetary gear sets, but such enlargement would not be acceptable to the automotive industry which places high priority on space and weight reduction.
Recently, the assignee of this invention has designed several new parallel-axis differentials with gear arrangements adaptable for C-clip assembly. (See U.S. Pat. No. 5,292,291 to S. E. Ostertag; U.S. Pat. No. 5,389,048 to L. E. Carlson; and my application Ser. No. 327,027, identified above). In these designs, C-clip assembly is facilitated by spacing the side gears apart along their common axis and by extending the length of straddle-type planet gears. In the differentials disclosed in the Ostertag patent and in my application Ser. No. 327,027, three pairs of planet gears are positioned at 120.degree. intervals around the common side gear axis.
In contrast, the Carlson patent provides clearance for C-clip assembly directly through the differential housing by organizing the planet gears in two sets of three or four planet gears, the sets being positioned 180.degree. apart about the common axis of the differential. In this latter reference, the planet gears of each set are in triplet or quadruplet meshing arrangements, each set having two outer planet gears in mesh with either a single center planet gear or with two center planet gears that are in mesh with each other. These latter Carlson designs combine accessible space for C-clip assembly with gear trains having torque bias equivalent to well-known differentials utilizing three planet gear pairs or four planet gear pairs positioned in 120.degree. or 90.degree. orientations, respectively.
However, the advantages of the just-described Carlson differential are offset by an uneven load distribution. Namely, each of the center planetary gears (i.e., of each triplet or quadruplet planetary combination) carries twice the load carried by each of the outer planet gears of each combination. Such uneven and excessive loading of the planet gears can result in unsatisfactory noise and wear problems.
My invention overcomes these problems and improves parallel-axis differential design (a) by accommodating C-clip assembly without significant increase in differential size and weight, and/or (b) by providing significant weight reduction and improved lubrication; and it accomplishes these improvements without reducing the differential's torque bias specifications.